Roofing material over shiplap

We're selling our house and the roof is at the end of its life - the house was built in the 20's and has shiplap underlayment with one layer of roofing material over it.

Our intention was to go over it with a new roof - now I'm hearing that some companies won't warranty over shiplap - can anyone address that and recommend options......we really can't afford to re-sheet and aren't even sure it's necessary - please help!

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Ship Lap is a profile of siding material, not roof sheathing. You sure that's what it is?
If it's really just 1 X 6's, it's shingled over all the time if any repairs are done to any broken boards before roofing.

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Other building materials (like board insulation) have a shiplapped edges, so it's not limited to siding and could have been used as roof decking. I imagine the siding is shiplapped as well. Regardless, assuming the boards are in good condition you should have no trouble. Due to age and the amount of roofs installed on them they might be more likely to split and crack, so some contractors prefer to cover the old decking with sheets of obs or plywood.